Self-luminous tritium light sources



United States Paten1: Oi

3,478,209 Patented Nov. 11, 1969 hee U.S. Cl. Z50- 77 4 Claims ABSTRACTOF THE DISCLOSURE This invention relates to self-luminous tritium lightsources with improved light output and improved longevity wherein theloss of tritium in the tritiated light source is minimized by preventingexchange of tritium with hydrogen in water vapor which may becomepresent in the tritium-activated lamp.

Lamps of the self-luminous type in which the light source comprises aphosphor which is excited by a radioactive material are known as betalights, and beta lights which use either gaseous or solid tritiatedluminous cornpounds can be considered as tritium beta lights.

It is a primary object of the present invention to increase the initiallight output of such tritium beta lights, and also to improve the lightoutput longevity of the same.

It is another object of the present invention to provide for aconstruction which minimizes the loss of tritium in a tritiated lightsource so as to improve the long term luminosity of the light source.

It is yet a further object of the present invention to provide sealedbeta ray light sources Iwith a minimum of residual water vapor in thesealed source so as to result in increased initial luminosity andincreased long term luminosity.

It is still anotnher object of the present invention to provide for theabove improvements in beta ray light sources, in particular in the betaray light source structure disclosed in my co-pending U.S. patentapplication Ser. No. 271,770, iiled Apr. 9, 1963.

Other objects and advantages of the present invention will be apparentfrom a further reading of the specification and of the appended claims.

With the above and other objects in view, the present invention mainlycomprises as a self-luminous light source, a sealed casing which is atleast partially transparent, and which is preferably of a materialresistant to darkening under beta ray bombardment, a phosphor which isexcited by beta rays to emit light, a source of tritium which gives offbeta rays which in turn excite the phosphor to emit light, and adehydrating agent within the casing to absorb Water vapor. Thedehydrating agent may be any dehydrating agent which is non-reactivewith any of the other materials in the casing, e.g. silica gel, calciumhydroxide, etc.

It is well-known that tritium (II-3) is an isotope of hydrogen, and ithas been found that there is an exchange of tritium with the hydrogen inwater or water vapor. Although this exchange is qiute low when oneconsiders the rabsolute concentrations of tritium in the air, theexchange assumes importance, if one considers tritiated radioactivelight sources.

It has been found according to the present invention that by providing aiield tritiated radioactive light source and further providing adehydrating agent such as silica gel therein, the exchange of tritiumfrom a tritiated light source with water vapor in the atmosphere and/ orin the eld tritiated radioactive light source is minimized, and inaddition, the residual water vapor pressure in the iield source isdecreased, so that the initial luminosity of the light source isincreased and the long term luminosity is also increased. The increasein initial luminosity results from the decrease in the residual watervapor pressure in the iield source which decreases the absorptivebarrier between the phosphor particles, for example the particles ofluminous zinc sulphide.

In accordance with the preferred embodiment of the present invention,the invention mainly comprises an improved radioactive light sourcewhich comprises a casing having disposed therein a radioactive regioncontaining a tritium-containing material giving oif beta rays, aphosphor region positioned in front of the tritium-containing region inthe direction of light discharged from the source, the phosphor regionbeing of suicient thickness to absorb a substantial portion of beta rayswithout substantial absorption of light rays, a light reflective andbeta ray reilective heavy metal reilecting region positioned behind theradioactive region, the heavy metal having an atomic number of at least45 and having a thickness suliicient to reflect beta rays and thereflecting region having a frontfacing light reflecting surface so thatthe reflecting region serves to reilect both light and beta raysforwardly, the forwardly directed beta rays exciting the phosphor regionand being converted into light, and a dehydrating agent located withinthe light source for the absorption of water Vapor within the lightsource and for the prevention of exchange of tritium with water vapor.

As employed throughout the specication and claims, the terms forward andfront are used to denote the areas or regions Ibetween the radioactivesource and the external environment to be illuminated. Similarly, theterm back region is employed to denote the area behind the radioactivematerial and away from the area or surface from which light is directedto the external environment.

In accordance with the present invention it is possible to providecapillary tubes of glass, vinyl plastics, styrene plastics, or the like,which are sealed and which contain the phosphor and either a gaseous orsolid source of tritium, and which contain the dehydrating agent, suchas silica gel either at the ends of the capillary tube or disposedthroughout the phosphor. The source of tritium may be a gaseous or solidtritiated luminous material such as tritiated zinc sulphide. Thecapillary tube may be entirely transparent or it may be partially opaqueto pro- Vide a split system, e.g. a dual or split source forspectrophotometric measurements.

The capillary tubes may be straight tubes or may be odd shapedstructures, for example in the shape of letters of the alphabet.

The tritium-luminous material capillaries provide a maximum luminosityper quantity of radioactive material and can be used in a powder,non-gaseous tritiated medium.

It is also possible according to the present invention to employ fillinggases which are dry and of low molecular weight (for example helium) inorder to improve the light output initially and transiently.

By the use of the dehydrating agent, for example, silica gel, at the endof the tube where it is heat sealed, it is possible to accomplish theheat sealing under atmospheric conditions, which minimizes thedeleterious effects of water vapor.

The back heavy metal reecting region which is used in the constructionof the preferred embodiment of the present invention should desirablyhave a high atomic number of the metal, i.e. at least 45 and preferablygreater than 76, in order to serve to back scatter the beta particles,

as well as to reflect light. The reflected beta particles then furtherexcite the forward phosphor regions and ultimately this energy isdischarged from the system in the form of light energy.

It is essential in this construction that the back scattering region becharacterized as a metal or metal composite having an atomic number ofat least 45 in order to reflect at least 60% of the back directed betarays. Materials of lower atomic number such as aluminum cannot serve toeffectively back scatter the beta particles. By use of a heavy metalregion beta particles which would normally be absorbed outside of thephosphor light producing material are more efciently utilized within thephosphor regions. Thus, it is possible by the use of this system to usemerely the heavy metal back scattering region and a front phosphorregion in yet higher efficiencies in terms of light conversion thanwould be effected by the use of a two layer phosphor system with orwithout the use of an aluminum reecting surface, as for example, shownin U.S. Patent No. 2,953,634 to MacHutchin et al.

This construction is particularly suited to the use of relatively weakbeta ray emitters such as tritium H-3, and the provision of thedehydrating or hygroscopic agent such as silica gel permits the maximumutilization of the tritium because of the minimizing of the exchange oftritium with hydrogen of water vapor.

In one aspect of the invention the tritium is deposited on anintermediate phosphor layer. This is desirable because the radiationfrom the tritium is maximally utilized and the upper non-activatedphosphor layer serves as a protective barrier against radioactivecontamination and mishandling.

Numerous types of phosphors or phosphor combinations such as zincsuldes, cadmium suldes, zinc silicates, zinc beryllium silicates, zincoxides, calcium tungstates, etc., are employed in the present structure.The depth of the front phosphor region will vary somewhat depending onthe energy level of the radioactive source but will be of suflicientdepth so as to absorb beta rays but not light rays. This is madepossible by the fact that the attenuation thickness of opticaltransmission is substantially greater than the beta ray thickness forcomplete absorption of weak beta rays from the radioactive source, e.g.,tritium. Thus the depth of the front phosphor region may be controlledto fall within a region giving at least 90% absorption of the weak betarays, without absorbing substantial quantities of the light ray. Thus,for example, when employing zinc sullide or cadmium sulde phosphors incombination with a tritium beta ray source, a thickness of l mil allows90% of the light rays to pass through unabsorbed while at least 90% ofthe weak beta rays are absorbed, since the absorption thickness for weakbeta rays is of the order of 1 0-20 microns (depending upon the natureof the absorber).

The average particle size of the phosphor preferably lies in the micronrange, eg., 2 to 30, especially 10 to 30 microns. This is desirablebecause there is little self attenuation of the light in thin layers.However, if particle size is too small there are large light scatterlosses.

As noted previously, it is essential that the back heavy metal region beof a metal or metal laminate having an atomic number greater than orequal to 45 in order to effectively return the beta rays to the forwardpart of the system and to eifectively convert their energy into light.Simultaneously the heavy metal reflects light forwardly, thus giving ahighly elfective overall conversion of radio-active energy to lightenergy. It is particularly preferred to employ platinum, osmium,iridium, and their alloys, as the heavy metal back reflecting region.Alternatively, bismuth or lead or high atomic weight oxides such as leadoxide can be employed. Additionally, a bound laminate of aluminumdeposited on a heavy metal such as platinum, bismuth, or lead can beutilized, the aluminum deposit serving to improve light reflection. Itshould be noted that the heavy metal region (compound of heavy metalwith or without bound aluminum) is positioned closest to the backphosphor region (if one be employed) and is separated from the frontphosphor region by the radioactive source. This is necessary since theback reflector region is employed to reflect both light and beta raysforwardly to the area where it is discharged from the structure in theform of light rays.

It is noted that since the front phosphor region is of suflicient depthso that at least some portions thereof are not radioactive, it serves asa protective cover absorbing the beta rays, as well as being a source oflight and thus no additional protective covers are necessary. Normally,however, it will be desirable to use a front glass or plastictransparent cover such as one made of methyl methacrylate or mica.However, no distinct radio-active absorbing protecting structure isrequired. It is desirable, however, to coat the internal surface of thetransparent cover with an anti-reflecting coating such as magnesiumuoride so as to minimize the internal reilection of the emitted lightrays and thus maximize the effective light sent outwardly to theexternal environment.

The various aspects and modifications of the present invention will bemade more clearly apparent by reference to the following description andaccompanying drawings, in which:

FIGURE l illustrates a capillary tube structure in accordance with thebasic concept of the present invention.

FIGURE 2 illustrates a system characterized by the use of a single frontphosphor region in combination with a heavy metal backed reflectingregion in which the dehydyrating agent is distributed in the phosphor.

FIGURE 3 illustrates the use of multiple phosphor regions in combinationwith a solid radioactive source.

FIGURE 4 depicts a system amenable to the utilization of a gaseousradioactive material in which the preferred embodiment of the presentinvention is combined with the use of a dehydrating agent.

lIn FIGURE l a capillary tube 11, which may be made of glass, or anysuitable transparent plastic such as a vinyl plastic or polystyrene,having a wall thickness of, for example, about 0.1-0.2 mm., the entirecapillary tube having a diameter of about 1 mm., is heat sealed at itsend 14. The tube is provided at the end 14 where it is heat sealed witha dehydrating agent 13, e.g. silica gel. As shown in FIGURE l, the tubecan be heat sealed at both ends 14 and 14', in `which case both ends areprovided with the dehydrating agent 13. This silica gel located at theend of the tube where it is heat sealed permits the sealing of the tubeunder atmospheric conditions and minimizes water vapor effects. Thephosphor 12, which may for example be tritiated zinc sulphide isdisposed throughout the tube. It is possible to have the dehydratingagent dispersed in the phosphor, as will be shown in the discussion ofFIGURE 2.

With reference to FIG. 2, shown therein is a system characterized by theuse of a front phosphor region and solid radioactive materials imbeddedin a phosphor layer, there being no distinct back phosphor regionemployed in the illustrated system. The entire system is enclosed incasing 1 which may be made of any of a wide variety of materials such asglass, plastics, methacrylates, epoxy resins and metals, such asaluminum or iron. Casing 1 in combination with transparent glass orplastic cover 5 provides an enclosure for containing the system of thepresent invention whereby beta rays are converted into light. The sourceof radioactivity in region 7 are radioactive particles imbedded in or onthe phosphor grains, which also have particles of silica gel dehydratingagent imbedded therein. The actual impregnation of the phosphor particlewith the radioactive solid can be done by a wide variety of conventionaltechniques, as for example, (a) sedimentation and evaporation, (b)vacuum evaporation, (c) slush milling and evaporation, (d) spraycoating, etc. The radioactive solid is a stearic type (or other organicor inorganic derivative) solid and a ZnS phosphor is employed. Theradioactive material gives off beta rays having an energy range between3 kev. to 17.9 kev. The phosphor particles preferably range between yland 24 microns in size and the radioactive material comprises about -6to 104% (by weight of the phosphor). Region 7 is approximately 5-18microns in depth.

lPositioned forwardly from said radioactive source is front phosphorregion 8. Region 8 may contain one or more layers of phosphor particleswhich are excited by the beta rays given off from region 7 and thusconvert the radioactive energy into light energy which passes outwardlythrough transparent cover 5. At least a substantial portion of region 8is free of radioactive materials so as to serve as a shield layer,preventing the weak beta rays from passing out through transparent cover5. By the same measure the width of phosphor layer 8 is such that thelight produced therein is not absorbed to a substantial degree and thuspasses out to the external source. Phosphor particles 4 may be the sametype of phosphor-containing material employed in the radioactive regionor alternatively can be a different phosphor-containing material, as forexample, in the present illustration, calcium tungstate. In general,there is no purpose for another phosphor in the coverage light source asanother phosphor would yield another color. However, two differentphosphors may be desirable where it is desired to obtain two colorpeaks, e.g. in the case of a double light standard source. In any event,either or both of the types of phosphors may have the dehydrating agentimbedded therein.

Number 6 in the drawing represents the radioactive substance depositedon or impregnated in phosphor particles 3. Numeral 9 in the drawingrepresents the dehydrating agent particles deposited on or imbedded inthe phosphor particles 3 and the phosphor particle 4.

Since the beta rays are being given off in a variety of directions,normally only those passing forwardly would be seen by light producingphosphor region 4. However, in accordance with the present invention,region 2 containing a heavy metal, i.e., a platinum layer, is positionedbehind the radioactive region 7 and serves to reflect both beta rays andlight which may be directed inwardly from phosphor regions 7 and 8. Thereflected light and back scattered beta rays are reflected forwardlyinto phosphor region 8 and are effectively made use of, the latter beingconverted to light energy upon impinging the phosphor particles, and theformer passing substantially unabsorbed out through transparent cover 5.In general, heavy metal reflecting region 2 will have a thickness ofapproximately 0.1 mil to l0 mils, preferably 0.1 to 2 mils, so as toeffectively serve to reflect beta ray particles. Thus, in the presentexample, region 2 will have a depth of about 0.5 mil; region 7, a depthof about microns and region 8, a depth of about 15-30 microns.Substantially no beta rays thus pass out of the system through cover 5while converting the beta rays of the radioactive solid source materialto light rays.

In general, it is desired that the various regions, e.g., phosphorregion, heavy metal reflecting regions be disposed in parallel relationin order to obtain uniformity of light discharged from the structure.While parallel curved surfaces can be employed, in general it isdesirable to employ relatively flat regions.

Turning to FIG. 3, shown therein is a particularly preferred embodimentof the present invention employing a plurality of phosphor regions incombination with a heavy metal reflecting region. The source of the betarays are zinc sulfide particles having a tritiated center (about 10-7 to10-3 weight percent tritium based in zinc sulfide). The centralradioactive solid source region is shown as a single layer of tritiatedzinc sulfide particles although a plurality of layers could, of course,be employed. Throughout the structure various binders, plasticizers,etc., can be employed to bind the various particles to each other or tosurfaces of the composite structure. Inorganic adhesives, such as sodiumsilicate and potassium silicate are particularly desirable because oftheir stability. Additionally, various resins such as epoxy resins orethyl-cellulose can be employed. The binders, plasticizers, etc., areindicated by the numeral 103 in the drawing.

A front phosphor particle region 109 is positioned between radioactivematerials 106 and the light discharging portion of the overallstructure. The present example phosphor region 109 contains one or morelayers of zinc sulfide phosphor particles 102. Particles 102 are 18microns average, in size. The depth of region 109 is about 18 microns.

Positioned behind the radioactive source is a second phosphor region 108similarly containing zinc sulfide particles. Beta rays given off by thetritium pass randomly and thus the presence of back phosphor layer 108serves to convert beta rays passed backwardly into light energy. Lightfrom regions 108 and 109, together with beta rays which are not emittedin a forward direction, strike heavy metal reflecting region 101 whichin the present example is a platinum reflector having a thickness of 0.5mil. The heavy metal serves to reflect `both the light and the betaparticles forwardly. The reflected beta particles then come into contactwith the phosphor in region 108 or 109 and are converted into lightenergy which passes out directly, or through reflection, through thefront surface of the light producing system. Instead of platinum, leadoxide, platinum-iridium alloy rhodium, etc. could be employed for region101. The phosphor particles are embedded with the dehydrating agent 110,such as silica gel.

A glass or a plastic, e.g. methyl methacrylate, cover 107 is normallyemployed at the front surface of the structure. Preferably the glass hasan internal antirellecting region 10S which may take the form ofmagnesium fluoride which has been previously deposited on the internalportions of the glass. The magnesium fluoride insures that emitted lightis not internally reflected into the central portions of the structure,but rather passes out through the glass covering plate. Enclosuresurrounding the light source may be made of Lucite or any of a widevariety of conventional materials.

The relative dimensions of the system are as follows:

Approximate depth of front phosphor region The tritium radioactivematerial has a radioactivity ranging from 2.5 millicurie/cm.2 to a fewhundred millicurie/cm2. By operating in accordance with the presentinvention a light brightness level (having a higher efficiency aspreviously stated) ranging from 5 microlamberts to a few hundredmicrolamberts is obtained. The efficiency of converting the beta raysinto light energy can be better than 2 microlamberts per millicurie ofsolid tritiated compound in the low level light range. This is based onphotometric measurements using an Aminco photomultiplier photometer andtritiated luminous standards.

FIG. 4 illustrates a structure particularly suitable for use in systemswherein a gaseous radioactive material, such as krypton-SS or tritium(H-3) are employed. The system of FIG. 3 is quite similar to FIG. 2 inthat it contains two phosphor particle regions, 202 and 203 positionedon each side of radioactive region 206. Normally region 206 is evacuatedthrough port 207 and thereafter radioactive gas is injected throughinlet 207 to reach the pressure desired. Normally atmospheric orsomewhat less than atmospheric pressure is utilized. Light source 200similarly contains a heavy metal back reflecting layer 201 which servesto refiect both light and beta rays forwardly, light ultimately passingthrough transparent cover 205. The phosphor particles may be of any of awide variety, eg., zinc sulfide, cadmium tungstate, etc. The thicknessof the front phosphor region in particular is chosen so as to absorbsubstantially all the beta rays emitted from region 206 in a forwarddirection while allowing the light generated by the excitement of thephosphor particles to pass outwardly. Structure 200 may be enclosed bywalls 204 which may be made of aluminum. A body of dehydrating agent 208such as silica gel, is provided at the walls 204 to minimize Water vaporeffects and prevent exchange of tritium with water vapor.

Cell 200 is gas-tight so that the effect of the dehydrating agent is atits maximum. In the present example, the space between phosphor regions203 and 202, i.e. the depth of the radioactive region 206 is of theorder of 1 centimeter, and the phosphor regions have an approximatedepth of about 18 microns. It is also to be noted that the overall depthof the cell, i.e. 1.5-3 centimeters is only a fraction of the otherdimensions of the cell, eg., length, 25 cm.; width, 7.5 cm.; and thusmaximum efficiency may be approached from the geometrical and reectiveproperties of the configuration.

It should be clearly understood that the present light source can beemployed in a variety of manners. They can be employed for railway andsignaling purposes. They find application as a lantern or as a marker orsign; when employing it for the latter purpose a portion of the coveringplate may be made opaque and so the transparent portion is illuminatedand produces a self-luminous form such as a traffic speed indicator ordirectional signal, portable map reader or negative X-ray copier orreader.

Various modifications may be made to the present invention. For the moreenergetic medium energy beta emitter such as Kr-85 (gaseous type) and`thallium204 (solid type) one may employ the basic combination of aheavy metal back-scatterer and light reflector coupled with a singlephosphor layer on the front fact to produce a more effective lightsource. In this case the light attenuation produced by both a front andback phosphor can be appreciable; hence one would Want maximumreflection of the beta rays.

With reference to the gas systems, one may utilize solely to take theform of a radioactive light source employing a weak beta ray source inWhich substantially planar regions of heavy metal reflector, phosphorparticles, and radioactive particles are utilized. The heavy metalregion serves both as an electron and light reiiector. A minimal numberof layers of a phosphorized material containing a radioactive source,i.e. tritiated phosphors can be employed with a front non-radioactivephosphor region serving as a source of light through *excitement by betarays as well as substantially absorbing all forwardly directed beta raysand insuring safety of the overall device. A

Having described the present invention, that which is sought to beprotected is set forth in the following claims.

What is claimed is:

1. An improved radioactive light source which comprises a casingdefining an interior chamber having disposed therein a radioactiveregion containing tritium which gives off beta rays, a phosphor regionpositioned in front of said radioactive region in the direction of lightdischarged from. said source, said phosphor region being of sufficientthickness to absorb a substantial portion of beta rays withoutsubstantial absorption of light rays, a light reflective and beta rayreflective heavy metal reiiecting region positioned behind and enclosingthe back portion of said radioactive region, said heavy metal having anatomic number of at least 45 and having a thickness sufficient toreflect beta rays, said metal being positioned adjacent to saidradioactive region in direct contact with the beta rays given off bysaid radioactive region and having a front-facing light reflectingsurface so that said reflecting region serves to reflect both light andbeta rays forwardly, said forwardly directed beta rays exciting saidphosphor region and being converted into light, and a dehydrating agentalso located in said chamber defined by said casing so that it is at alltimes in direct contact with the gases therein, said dehydrating agentthus absorbing water vapor and minimizing exchange of tritium with thehydrogen of water vapor.

2. A radioactive light source structure comprising a casing defining aninterior chamber having disposed therein a radioactive region containingtritium beta emitters, a phosphor region `positioned between saidradioactive region and the area wherein light is discharged from saidstructure, said phosphor region being of sufiicient depth to absorb atleast of the weak beta rays emitted from said radioactive region withoutsubstantially absorbing light rays, a light reflective and beta rayreliective heavy metal reliecting region positioned behind and enclosingthe portion of said radioactive region away from the area of lightdischarged from said structure, said heavy metal region comprising ametal having an atomic number of at least 45 and being of asuicient'thickness to back scatter a major portion of the beta rayscontacting its structure, said kmetal portion being adjacent saidradioactive region and in direct contact with the beta rays given off bysaid radioactive region and having a forwardfacing light refiectivesurface so as to reect both light and beta rays forwardly, saidreflected beta rays and beta rays emanating from said radioactive regionserving to excite the phosphor region and be converted into lightenergy, and a dehydrating agent also located in said chamber defined bysaid casing so that it is at all times in direct contact with the gasestherein, said dehydrating agent thus absorbing and minimizing exchangeof tritium with the hydrogen of water vapor.

3. The structure of claim 1, wherein said dehydrating agent is dispersedin said phosphor region.

4. The light source of claim 1 wherein said dehydrating agent is silicagel.

References Cited UNITED STATES PATENTS 3,176,132 3/1965 Muller Z50-106 X3,260,846 7/1966 Feuer 250--106 X ARCHIE R. BORCHELT, Primary ExaminerU.S. C1. X.R.

